The present invention relates to a highly accurate and precise linear friction drive for a dosage metering device particularly adapted for operating syringes or pipetters. In particular, the invention comprises a drive shaft and a circular drive shaft. The driven shaft has a flat side pressed against the drive shaft. When the drive shaft is rotated, the driven shaft is moved by friction and with negligible slip. The driven shaft and the drive shaft are analogous to a toothless rack and pinion set. The rack is rollably mounted on a set of guide rollers opposed to and straddling the pinion. One of the guide rollers is fixed, and one is moveable. The latter is mounted on a spring-loaded swing arm providing mechanical advantage. A force exerted against the swing arm is multiplied and transferred to the pinion via the rack.
In instruments useful for the automatic analysis of physiological fluids and the like, small quantities of such fluids are diluted with relatively large quantities of reagents. These devices accurately aspirate physiological fluids and reagents by means of a plunger-driven syringe and deliver the materials by means of communicating tubing terminated in a pipetting tip into a receptacle such as a dilution cup. For a variety of reasons, it is important that the proportions of physiological fluid and reagents be accurate. One obvious reason is that the chemical analysis will change if the proportions of the physiological fluid and reagents are not consistent.
Instruments used for the purposes described above are known. For the most part, such instruments use belt or gear drives for actuating the plunger. Such drives normally have means for positively stopping the motion of the plunger, for example, a mechanical stop. Although reasonably satisfactory for measuring selected incremental values of reagents, such systems do not have flexibility, nor are they as accurate and precise and is now achievable by means of the present invention.
Gear drives may be substituted for the belt drive mentioned above. However, even the most accurate gears, sometimes referred to as antibacklash gears, suffer from what is known as cyclical error. Although the maximum value of such error is very small, it is not consistently the same. It varies over the cycle of gear movement. At one time in the cycle cyclical error may be 0.1%, and another time it may be as high as 5%. The problem is that one cannot correct for the variation in the error. Further, errors are disproportionate. If one wished to mix two measures of liquid, for example, 5 .mu.l and 200 .mu.l, a 0.5 .mu.l error relative to the smaller quantity of liquid would be 10%. The same error with respect to the larger quantity would amount to only 0.5%. Thus, the error in the smaller amount is 20 times larger than the error in the larger amount.
It has been found that the use of a friction drive, analogous to a toothless rack and pinion, results in a proportionate, linear error. In other words, any motion of the rack relative to the pinion results in the same percentage of error. If the example above, if a 5 .mu.l sample were to be mixed with a 200 .mu.l sample, each might see an error of 1%. Thus, because the error percentage is the same, the proportional relationship between the two amounts remains about the same. Because the error is linear, it is predictable and it may be corrected by simple calculation.
Friction drives have been rejected in the past because they slip. In particular, the prior art friction drives focused on high power levels with significant slip. The present invention provides means for minimizing the slip and other sources of error, especially cyclically varying errors, in order to produce a highly accurate and precise linear fluid metering system.
The relative ease with which parts move with respect to each other is sometimes referred to as mechanical slop. In a machine designed for accuracy, mechanical slop introduces errors. The moving parts, hereinafter described, are stiffly mounted to prevent mechanical slop. For example, the moveable guide roller and swing arm are secured for preventing lateral deflection of the guide roller and rack supported thereby. The guide rollers are double row, angular, preloaded, radial ball bearings of a known kind. They have working surfaces (not shown) in opposition for resisting lateral motion of the guide roller in relation to its mounting. Accuracy of the present invention is enhanced because slop is reduced.
The present invention utilizes a digital stepping motor governed by a pulse width modulation control system. The digital stepping motor drives the pinion in very small steps so that accurate positioning of the rack may be achieved. Thus, fixed incremental stops and the consequent lack of flexibility are eliminated because the rack may be positioned in virtually any location.